1. Field of the Invention
The present invention relates to an excitation control apparatus of a synchronous generator used to improve the voltage stability of an electric power system.
2. Description of Related Art
FIG. 5 is a constitutional block diagram of a conventional excitation control apparatus of a synchronous generator.
In FIG. 5, a reference numeral 1 indicates a transmission voltage setting circuit for setting a transmission voltage of a power transmission system (not shown) to a reference value VHref. The transmission voltage of the power transmission system relates to a high voltage on a high voltage side of a transformer (not shown) arranged between a synchronous generator (not shown) and the power transmission system. A reference numeral 2 indicates a subtracting unit for subtracting a transmission voltage VH actually applied to the power transmission system from the reference value VHref of the transmission voltage set in the transmission voltage setting circuit 1, a reference numeral 3 indicates a tap position detecting circuit for detecting a tap position of the transformer, and a reference numeral 4 indicates a voltage-up ratio calculating circuit for calculating the inverse value of a voltage-up ratio n of the transformer from the tap position of the transformer detected in the tap position detecting circuit 3 and multiplying the reference value VHref of the transmission voltage VH by the inverse 1/n of the voltage-up ratio n.
A reference numeral 5 indicates a reactive current setting circuit for setting a reference value of a reactive current Iq of the synchronous generator to a value Iq0, a reference numeral 6 indicates a multiplier for multiplying the reference value Iq0 of the reactive current Iq set in the reactive current setting circuit 5 by a rated reactance value Xt of the transformer, a reference numeral 7 indicates an adding and subtracting unit for adding a multiplied result V.sub.Href /n of the voltage-up ratio calculating circuit 4 and a multiplied result Iq0*Xt of the multiplier 6 together and subtracting a reference value V.sub.gref of a terminal voltage Vg of the synchronous generator from an added result V.sub.Href /n+Iq0*Xt, a reference numeral 8 indicates a multiplier for multiplying an added-subtracted result V.sub.Href /n+Iq0*Xt-V.sub.gref of the adding and subtracting unit 7 by a divided result .beta./KH obtained by dividing a gain reducing coefficient .beta. by a gain KH of a transmission voltage control, a reference numeral 9 indicates an adder for adding a subtracted result V.sub.Href -V.sub.H of the subtracting unit 2 and a multiplied result .beta./KH*(V.sub.Href /n+Iq0*Xt-V.sub.gref) of the multiplier 8, a reference numeral 10 indicates a multiplier for multiplying an added result V.sub.Href -V.sub.H +.beta./KH*(V.sub.Href /n+Iq0*Xt-V.sub.gref) of the adder 9 by the gain KH of the transmission voltage control, and a reference numeral 11 indicates a signal producing unit for multiplying a deviation (V.sub.gref -Vg) between the reference value V.sub.gref of the terminal voltage Vg of the synchronous generator and the terminal voltage Vg by the gain reducing coefficient .beta. to obtain a multiplied result .beta.*(V.sub.gref -Vg) and adding this multiplied result .beta.*(V.sub.gref -Vg) and a multiplied result KH(V.sub.Href -V.sub.H)+.beta.*(V.sub.Href /n+Iq0*Xt-V.sub.gref) of the multiplier 10 to obtain an added result. PA1 However, in cases where the tap position of the transformer is changed, though the voltage-up ratio n of the transformer and the reactance value of the transformer are changed, because the transmission voltage V.sub.H is controlled by using the gain KH set to a constant value, there is a drawback that the transmission voltage V.sub.H becomes instable. In detail, there are following drawbacks. PA1 In cases where an accident of a wiring or a sharp increase of a load occurs, the transmission voltage V.sub.H of the power transmission system is sharply lowered, i.e., droops. In this case, it is required to set the terminal voltage Vg of the synchronous generator to a higher value for the purpose of maintaining the transmission voltage V.sub.H. PA1 In this case, when the tap position of the transformer is changed, because the voltage-up ratio n of the transformer and the reactance value of the transformer are changed, a voltage drooping factor in the transmission voltage control, which denotes the coefficient n*.beta.*Xt/(.beta.+n*KH) of the second term (Iq-Iq0), changes. Therefore, in cases where a plurality of synchronous generators are connected with the same bus of the power transmission system through a transformer and are operated in parallel to each other, because the transmission voltage V.sub.H to be controlled is common to the synchronous generators, when a transmission voltage characteristic of each synchronous generator is changed, the reactive currents Iq of the synchronous generators are put out of balance. Therefore, there is a first drawback that the instability of the transmission voltage V.sub.H of the power transmission system easily occur. PA1 Also, in cases where one core-type transformer, of which the tap position is set to a rated position, is connected with two synchronous generators, even though the reactance (or the reactance changing ratio nr) of the core-type transformer for one synchronous generator agrees with that for the other synchronous generator, when the tap position of the core-type transformer is changed, the reactance changing ratio nr of the core-type transformer for one synchronous generator is increased or decreased in the opposite to the decrease or increase of the reactance changing ratio nr of the core-type transformer for the other synchronous generator. In this case, the reactive currents Iq of the two synchronous generators connected with the core-type transformer are changed in the opposite direction to each other, and the voltage drooping factors for the two synchronous generators in the transmission voltage control are changed in the opposite direction from each other. Therefore, there is a second drawback that the transmission voltage control becomes instable and the reactive currents Iq of the two synchronous generators are put out of balance in case of the core-type transformer. PA1 tap position detecting means for detecting a tap position of a transformer arranged between a synchronous generator and a power transmission system; PA1 drooping gain calculating means for calculating a drooping gain of a transmission voltage control according to the tap position of the transformer detected by the tap position detecting means to maintain a drooping factor of the power transmission system in the transmission voltage control to a constant value; and PA1 voltage control means for controlling a transmission voltage of the power transmission system according to the drooping gain calculated by the drooping gain calculating means and a prescribed value. PA1 an operation value setting circuit for setting a drooping factor of the power transmission system in the transmission voltage control; PA1 a voltage-up ratio calculating circuit for calculating a voltage-up ratio of the transformer from the tap position of the transformer detected by the tap position detecting means; and PA1 a drooping gain calculating circuit for calculating the drooping gain in the transmission voltage control from the drooping factor set by the operation value setting circuit and the voltage-up ratio calculated by the voltage-up ratio calculating circuit. PA1 an operation value setting circuit for setting a drooping factor of the power transmission system in the transmission voltage control and setting the reference value of the reactive current of the synchronous generator; PA1 a voltage-up ratio calculating circuit for calculating a voltage-up ratio of the transformer from the tap position of the transformer detected by the tap position detecting means; PA1 a reactance changing ratio calculating circuit for calculating a reactance changing ratio of the transformer from the tap position of the transformer detected by the tap position detecting means; and PA1 a drooping gain calculating circuit for calculating the drooping gain in the transmission voltage control from the drooping factor set by the operation value setting circuit, the voltage-up ratio calculated by the voltage-up ratio calculating circuit and the reactance changing ratio calculated by the reactance changing ratio calculating circuit. PA1 reactive current detecting means for detecting an reactive current of the synchronous generator; and PA1 compensating means for compensating a reference value of the reactive current of the synchronous generator for the change of the tap position by using the reactive current of the synchronous generator detected by the reactive current detecting means and the tap position detected by the tap position detecting means, the reference value being used as the prescribed value by the voltage control means. PA1 an operation value setting circuit for setting a drooping factor of the power transmission system in the transmission voltage control and setting the reference value of the reactive current of the synchronous generator; PA1 a voltage-up ratio calculating circuit for calculating a voltage-up ratio of the transformer from the tap position of the transformer detected by the tap position detecting means; and PA1 a drooping gain calculating circuit for calculating the drooping gain in the transmission voltage control from the drooping factor set by the operation value setting circuit and the voltage-up ratio calculated by the voltage-up ratio calculating circuit. PA1 tap position detecting means for detecting a tap position of a transformer arranged between a synchronous generator and a power transmission system; PA1 reactive current detecting means for detecting an reactive current of the synchronous generator; PA1 compensating means for compensating a reference value of the reactive current of the synchronous generator for the change of the tap position by using the reactive current of the synchronous generator detected by the reactive current detecting means and the tap position detected by the tap position detecting means; and PA1 voltage control means for controlling a transmission voltage of the power transmission system according to the reference value of the reactive current compensated by the compensating means.
In the above configuration, an operation of the conventional excitation control apparatus is described.
In the example shown in FIG. 5, the added result of the signal producing unit 11 is output to an auto-voltage regulating unit AVR (not shown), and the transmission voltage VH of the power transmission system is controlled according to the added result to make the transmission voltage VH agree with the reference value VHref.
In cases where a transmission voltage control is performed to control the transmission voltage VH of the power transmission system to a constant value, the transmission voltage VH is expressed according to an equation (1). EQU VH=VHref-n*.beta.*Xt/(.beta.+n*KH)*(Iq-Iq0) (1)
Here, in the reactive current setting circuit 5, the reactive current Iq of the synchronous generator is set to the reference value Iq0 in cases where the transmission voltage VH agrees with the reference value VHref.
Therefore, as is apparently indicated in the equation (1), in cases where the synchronous generator is operated on condition that the reactive current Iq of the synchronous generator almost agrees with the reference value Iq0, the transmission voltage VH of the power transmission system can be maintained to the reference value VHref or a value near to the reference value VHref.
In contrast, in cases where the terminal voltage Vg of the synchronous generator is controlled to a constant value, the transmission voltage VH is expressed according to an equation (2). EQU VH=n*Vgref-nr*Iq*Xt (2)
Here, the symbol nr denotes a reactance change ratio of the transformer.
As is apparently indicated by comparing the equations (1) and (2) with each other, in the transmission voltage control in which the transmission voltage VH of the power transmission system is controlled to a constant value, as compared with a terminal voltage control in which the terminal voltage Vg of the synchronous generator is controlled to a constant value, a possibility of the decrease of the transmission voltage caused by a reactance change of the transformer is low, and an adverse influence of the change of the reactive current Iq is low. Therefore, the transmission voltage VH of the power transmission system can be stabilized.
Also, in the terminal voltage control in which the terminal voltage Vg of the synchronous generator is controlled to a constant value, when the voltage of the power system is lowered, the reactive current Iq of the synchronous generator is increased, and the voltage VH on the high voltage side of the transformer is lowered in proportion to the system voltage. In contrast, in the transmission voltage control in which the transmission voltage VH of the power transmission system is controlled to a constant value, even though the system voltage of the power system is lowered, because a lowering degree of the reactance of the transformer is recovered by heightening the terminal voltage vg of the synchronous generator, the lowering of the voltage on the high voltage side of the transformer can be prevented.
Accordingly, because the conventional excitation control apparatus of the synchronous generator is constituted as is described above, the voltage stability of the power transmission system can be effectively improved.
First Drawback:
However, when the terminal voltage Vg of the synchronous generator exceeds its upper limit, an auto-tap-control function OLTC of the transformer is normally operated, and a tap value of the transformer is changed to a tap value of a loading operation. As a result, the voltage-up ratio n of the transformer is heightened, and the terminal voltage Vg of the synchronous generator is returned to its rated value or a value near to the rated value.
Second Drawback:
As a type of three-winding-wire transformer frequently used, a shell type transformer and a core-type transformer are known. Characteristics of the shell type transformer are similar to those of a split type wiring transformer. That is, because the voltage-up ratio n depending on the tap position changes in proportion to the reactance changing ratio nr in the shell type transformer, the relationship between the transmission voltage V.sub.H of the power transmission system and the terminal voltage Vg of the synchronous generator is expressed according to an equation (3).
VH=n*Vg-n*Xt*Iq (3)
Also, in cases where the transmission voltage control is performed to control the transmission voltage VH of the power transmission system to a constant value, the transmission voltage VH is expressed in the same manner as in the equation (1).
However, in case of the core-type transformer, because the change of the voltage-up ratio n depending on the tap position is not proportional to the change of the reactance changing ratio nr, the relationship between the transmission voltage VH of the power transmission system and the terminal voltage Vg of the synchronous generator is expressed according to an equation (4). EQU VH=n*Vg-nr*Xt*Iq (4)
Therefore, in cases where the transmission voltage VH of the power transmission system is controlled to a constant value, the transmission voltage VH is expressed according to an equation (5). EQU VH=VHref-.beta.*Xt/(.beta.+n*KH)*(nr*Iq-n*Iq0) (5)
In this equation (5) for the core-type transformer, in cases where the transmission voltage VH of the power transmission system agrees with the reference value VHref, Iq=(n/nr)*Iq0 is satisfied. In other words, even though the transmission voltage VH is controlled to agree with its reference value VHref, the synchronous generator is operated while undesirably deviating the reactive current Iq of the synchronous generator from its reference value Iq0 by a value (n/nr-1)*Iq0 relating to the coefficient (n/nr).